European specifications for diesel fuel (Euro 4) require a high Cetane number (CN) (minimum 51) with a low specific gravity (0.845 maximum). This fuel can be readily produced from biomass, natural gas or coal via Fischer-Tropsch (F-T) synthesis. These approaches produce a diesel fuel that is very paraffinic with high CN and low specific gravity. Such diesel fuels, however, have relatively low energy content per gallon, and thus provide lower mileage per gallon when used as a transportation fuel. Additionally, in order to produce a diesel fuel having acceptable cold flow properties for use in many parts of the world, the F-T product needs to be a hydroisomerized. Other important disadvantages of F-T processes for producing diesel fuel from coal are that they have low thermal efficiency and relatively high CO2 emissions.
The overall thermal efficiency (HHV), for a balanced F-T plant with zero imported or exported power, is typically 41 to 48% depending upon the coal and gasification process utilized. The Chinese coal company Shenhua, which has built both commercial F-T and DCL plants, reported a thermal efficiency of 41.26% (1) for a Chinese coal using low temperature F-T in a recent publication. Synfuels China (1) reported a thermal efficiency from coal of 43%. In a presentation to NETL, Schmetz (7) reported (source: the National Coal Council) the thermal efficiency for an F-T plant with gas recycle to be 47.4%.
Some U.S. studies have calculated somewhat higher thermal efficiencies for F-T plants that utilize entrained gasifiers such as Shell and coproduce power (Polygen). Headwaters (2) reported a thermal efficiency of 48.4% for a recycle F-T plant design producing 399 MW of export power and 70 KB/D of naphtha and diesel and 47.4% for a Once Through (OT) F-T plant producing 1,139 MW of power and 70 KB/D of gasoline and diesel.
Direct Coal Liquefaction (DCL) has also been proposed as a route to produce diesel fuel. Diesel from DCL has excellent cold flow properties but specific gravity (0.86 to 0.90) is significantly above the Euro 4 specification and CN is lower than the Euro 4 specification, even after severe hydrotreating to remove heteroatoms and hydrogenate aromatics to naphthenes. Chevron (10) reported studies on diesel fuel production from direct coal liquefaction products. The CN of the diesel fuels produced ranged from 32.7 to 48.7.
Selectivity to diesel has been also generally lower for DCL than for Fischer-Tropsch plants. In the Headwaters balanced plant cases, selectivity to diesel on a C5+ LV % basis is 71%; whereas, Synfuels China reported an 80% selectivity to diesel on a C5+ basis from their F-T Demo Plant.
Based on back-to-back comparisons of F-T and DCL, DCL has a significantly higher thermal efficiency than F-T. In 2008, Headwaters (2) reported thermal efficiency of 60.1% for DCL versus the previously reported 47.4% and 48.4% for OT and Recycle F-T with power export, a 24 to 27% advantage for DCL versus F-T. In 2005, Lepinski (3) presented a comparison of DCL and F-T for a sub-bituminous coal. For a 50 KB/D plant, 32 KST/D of as received coal was required for the F-T plant and 23 KST/SD for the DCL plant; an increase of 39%. Shenhua (6) reported the thermal efficiency of DCL at 59.75% versus 41.56% for High Temperature F-T and 41.26% for Low Temperature F-T, an increase of 44%.
Companies active in the field have generally sought to increase coal conversion in DCL (7) on a once through basis in order to decrease capital cost. Typical reported DCL coal conversion, on a moisture and ash free (MAF) basis, has increased from about 70% in 1980 to over 90% by 1994. This trend has decreased capital cost but has required the use of more expensive low ash and high reactivity (2) coals in order to avoid higher levels of ash in liquefaction, fractionation, and PDX that would interfere with the DCL process. For example, currently available PDX units are unable to process bottoms having an ash higher than about 30%. Higher reactivity coals are generally high in vitrinite and low in inertinite.
The reported DCL coal conversion units require the use of a hydrotreator for preparing a donor solvent that is fed back to the input of the DCL unit to act as a solvent for the coal being converted and to provide additional hydrogen to the liquefaction process. The hydrotreating makes solvent less aromatic, which reduces its compatibility with coal. Additionally, the specific gravity of such donor solvents is low which makes it more difficult to prepare a stable slurry, and results in settling of ash and separation of a separate phase that can result in equipment deposits and plugging. The temperature of the prepared coal slurry is typically about 100° F.
Others have proposed combining Fischer Tropsch and Direct Coal Liquefaction units in a single plant to produce a blended product. In both the Gray (11) and Headwaters studies (2), the DCL and F-T processes were described as being operated in parallel, each receiving a coal feed, and the DCL and F-T products were blended on a 50:50 basis. Because of the lower thermal efficiency of F-T versus DCL, this requires that greater than 50% of the coal be processed by the lower thermal efficiency F-T. For Headwaters, (2) this was calculated to result in a Hybrid DCL/ICL plant that has a C3+ selectivity of 67% and a thermal efficiency of 58.7%. Neither the Gray nor the Headwaters combined FT and DCL plant has actually been constructed, and the descriptions say nothing about the CN of any diesel fuel that might be produced.
Currently, Carbon Capture and Sequestration (CCS) is the leading option for managing CO2 emissions. CO2 which is captured in the plant is compressed to approximately 2,400 psi and injected into underground formations. Because a significant portion of the CO2 produced by the plant, e.g., that emitted from furnaces, cannot be readily captured, as a practical matter the CO2 sequestered is limited to at most approximately 90%, and typically substantially less. In addition, depending on the environmental and geological conditions in the vicinity of the plant, the compressed CO2 may have to be transported substantial distances to the injection site, and the compression of the CO2 consumes energy, both of which lower the overall efficiency of the plant. Moreover, there is a continuing concern that the sequestered CO2 may escape into the atmosphere over time, thereby obviating at least part of the benefit of injecting the CO2. No provision for reducing CO2 emissions was made in any of the plants discussed above. Doing so would further reduce the thermal efficiency of the plants.